[TimTheTinker]

We're not using any Web Components APIs, thus no slow polyfills are required (see my comment for how we isolate component CSS).

As for eschewing the style cascade, yes, that is happening when we isolate each component's CSS. However, wouldn't using Web Components proper have the same problem? And yes--it would be a very large task to add a dark mode, as you say. Each component would have to be visited. However, when we drop IE11 support, CSS custom properties will make this potentially much easier.

We don't copy-and-paste CSS often (remember we're re-using components and composing them everywhere). If there's enough commonality for CSS copy/paste to be tempting, it's probably time to either revisit how components are being composed (do we need to create a new component, or maybe have these 2 similar components inherit from a base JS class?), or add those styles to a top-level stylesheet and use shared CSS classes.